US8692325B2 - Semiconductor device and method of manufacturing the same - Google Patents

Semiconductor device and method of manufacturing the same Download PDF

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US8692325B2
US8692325B2 US12/754,238 US75423810A US8692325B2 US 8692325 B2 US8692325 B2 US 8692325B2 US 75423810 A US75423810 A US 75423810A US 8692325 B2 US8692325 B2 US 8692325B2
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trench
region
wall portion
semiconductor substrate
semiconductor device
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US20100270616A1 (en
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Shinichiro Yanagi
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Renesas Electronics Corp
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    • H01L21/8232—Field-effect technology
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    • H01L21/8238—Complementary field-effect transistors, e.g. CMOS
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Definitions

  • the present invention relates to a semiconductor device and a method of manufacturing the same, and particularly to a semiconductor device in which a gate electrode is partly located over a buried insulating film and a method of manufacturing the same.
  • LDMOS Laterally Diffused Metal Oxide Semiconductor
  • REduced SURface Field (RESURF) MOS transistor As a typical structure of a high-breakdown-voltage Laterally Diffused Metal Oxide Semiconductor (LDMOS) transistor, there is a REduced SURface Field (RESURF) MOS transistor (see Patent Document 1). To provide the LDMOS transistor with a higher breakdown voltage, it is necessary to uniformly extend a depletion layer in the drift region of a drain and, as a technique therefor, there is a field plate effect using a gate electrode layer.
  • the field plate effect is obtained by extending the gate electrode layer toward a drain over a semiconductor substrate, but a high electric field is formed under the drain-side edge of the gate electrode layer. Accordingly, in a typical structure, a thick oxide film is formed over a surface of the semiconductor substrate, and the drain-side edge of the gate electrode layer is positioned over the oxide film, thereby reducing the intensity of the electric field under the drain-side edge of the gate electrode layer.
  • Non-Patent Document 2 As the thick oxide film described above, various oxide films are used depending on the generation of a process for forming the LDMOS transistor, but a Local Oxidation of Silicon (LOCOS) oxide film and a Shallow Trench Isolation (STI) are typically used.
  • LOCOS Local Oxidation of Silicon
  • STI Shallow Trench Isolation
  • a configuration using the STI is described in, e.g., Non-Patent Document 2.
  • a STI is mostly used as a thick oxide film.
  • An edge portion of the STI is steeper than an edge portion of a LOCOS oxide film so that current concentration occurs in the edge portion of the STI during an ON operation.
  • the current concentration causes an increase in impact ionization ratio, and may cause the degradation of electric characteristics due to the trapping of electrons.
  • the present invention has been achieved in view of the foregoing problem, and an object of the present invention is to provide a semiconductor device in which the degradation of electric characteristics can be inhibited and a method of manufacturing the same.
  • a semiconductor device includes a semiconductor substrate, an insulating film, and a gate electrode layer.
  • the semiconductor substrate has a main surface, and a trench in the main surface.
  • a buried insulating film is buried in the trench.
  • the trench has one wall surface and the other wall surface which oppose each other.
  • the gate electrode layer is located at least over the buried insulating film.
  • the trench has an angular portion located between the main surface of at least either one of the one wall surface and the other wall surface and a bottom portion of the trench.
  • an angular portion located between the main surface of at least either one of the one wall surface and the other wall surface and a bottom portion of the trench does not include an edge portion located at the intersection of either one of the one wall surface and the other wall surface and the bottom portion of the trench.
  • the angular portion is provided between the main surface of at least either one of the one wall surface and the other wall surface and the bottom portion of the trench. Therefore, it is possible to reduce electric field concentration on the wall surfaces of the trench, and inhibit the degradation of electric characteristics.
  • FIG. 1 is a plan view schematically showing a configuration of an analog/digital mixed chip
  • FIG. 2 is a cross-sectional view schematically showing a configuration of a semiconductor device in Embodiment 1 of the present invention
  • FIG. 3 is an enlarged cross-sectional view of a trench in the STI structure shown in FIG. 2 , which is for illustrating the cross-sectional shape of the trench;
  • FIG. 4 is a cross-sectional view schematically showing a first step of a method of manufacturing the semiconductor device in Embodiment 1 of the present invention
  • FIG. 5 is a cross-sectional view schematically showing a second step of the method of manufacturing the semiconductor device in Embodiment 1 of the present invention.
  • FIG. 6 is a cross-sectional view schematically showing a third step of the method of manufacturing the semiconductor device in Embodiment 1 of the present invention.
  • FIG. 7 is a cross-sectional view schematically showing a fourth step of the method of manufacturing the semiconductor device in Embodiment 1 of the present invention.
  • FIG. 8 is a cross-sectional view schematically showing a fifth step of the method of manufacturing the semiconductor device in Embodiment 1 of the present invention.
  • FIG. 9 is a cross-sectional view schematically showing a sixth step of the method of manufacturing the semiconductor device in Embodiment 1 of the present invention.
  • FIG. 10 is a cross-sectional view schematically showing a seventh step of the method of manufacturing the semiconductor device in Embodiment 1 of the present invention.
  • FIG. 11 is a cross-sectional view schematically showing an eighth step of the method of manufacturing the semiconductor device in Embodiment 1 of the present invention.
  • FIG. 12 is a cross-sectional view schematically showing a ninth step of the method of manufacturing the semiconductor device in Embodiment 1 of the present invention.
  • FIG. 13 is a cross-sectional view schematically showing a tenth step of the method of manufacturing the semiconductor device in Embodiment 1 of the present invention.
  • FIG. 14 is a cross-sectional view schematically showing a configuration of a semiconductor device according to a comparative example which does not have an angular portion at a sidewall of a trench of a STI structure;
  • FIG. 15 is a view showing the result of variations in drain current Ids with a lapse of time under an OLT test
  • FIG. 16(A) is a distribution diagram of an electron/current density in the comparative example under OLT stressing conditions
  • FIG. 16(B) is a distribution diagram of an electron/current density of a stepped type in the configuration shown in FIG. 2 under OLT stressing conditions;
  • FIG. 17(A) is a distribution diagram of an impact ionization ratio in the comparative example under OLT stressing conditions
  • FIG. 17(B) is a distribution diagram of an impact ionization ratio of a stepped type in the configuration shown in FIG. 2 under OLT stressing conditions;
  • FIG. 18 is a view showing the distribution of the impact ionization ratio along the interface of a semiconductor substrate between the points P 1 and P 2 of FIG. 17 in each of the configuration of the comparative example and the configuration (of the stepped type) shown in FIG. 2 , each under OLT stressing conditions;
  • FIG. 19 is a view showing the Vg-Id characteristic of a gate voltage Vg and a drain current Ids when charges are generated in an edge portion of a STI structure;
  • FIG. 21 is a cross-sectional view schematically showing a configuration of a semiconductor device in Embodiment 2 of the present invention.
  • FIG. 22 is a cross-sectional view schematically showing a first step of a method of manufacturing the semiconductor device in Embodiment 2 of the present invention.
  • FIG. 23 is a cross-sectional view schematically showing a second step of the method of manufacturing the semiconductor device in Embodiment 2 of the present invention.
  • FIG. 24(A) is a distribution diagram of an electron/current density in a comparative example under OLT stressing conditions
  • FIG. 24(B) is a distribution diagram of an electron/current density of a stepped type in Embodiment 2 under OLT stressing conditions
  • FIG. 25(A) is a distribution diagram of an impact ionization ratio in the comparative example under OLT stressing conditions
  • FIG. 25(B) is a distribution diagram of an impact ionization ratio of a stepped type in Embodiment 2 under OLT stressing conditions
  • FIG. 26 is a view showing the distribution of the impact ionization ratio along the interface of a semiconductor substrate between the points P 3 and P 4 of FIG. 25 in each of the configuration of the comparative example and the configuration (of the stepped type) shown in FIG. 21 , each under OLT stressing conditions;
  • FIG. 27 is a cross-sectional view schematically showing a configuration of a semiconductor device in Embodiment 3 of the present invention.
  • FIG. 28 is a cross-sectional view schematically showing a first step of a method of manufacturing the semiconductor device in Embodiment 3 of the present invention.
  • FIG. 29 is a cross-sectional view schematically showing a second step of the method of manufacturing the semiconductor device in Embodiment 3 of the present invention.
  • FIG. 30(A) is a distribution diagram of an electron/current density of a conventional type in a comparative example under OLT stressing conditions
  • FIG. 30(B) is a distribution diagram of an electron/current density of a stepped type in Embodiment 3 under OLT stressing conditions
  • FIG. 31(A) is a distribution diagram of an impact ionization ratio of the conventional type in the comparative example under OLT stressing conditions
  • FIG. 31(B) is a distribution diagram of an impact ionization ratio of the stepped type in Embodiment 3 under OLT stressing conditions
  • FIG. 32 is a view showing the distribution of the impact ionization ratio along the interface of a semiconductor substrate between the points P 5 and P 6 of FIG. 31 in each of the configuration of the comparative example and the configuration (of the stepped type) shown in FIG. 27 , each under OLT stressing conditions;
  • FIG. 33 is a cross-sectional view schematically showing a configuration of a semiconductor device in Embodiment 4 of the present invention.
  • FIG. 34 is a cross-sectional view schematically showing a configuration of a semiconductor device in Embodiment 5 of the present invention.
  • FIG. 35 is a view showing variations in drain current when an angular portion is provided at the source-side sidewall of a trench of a STI structure, and the position of the angular portion is varied;
  • FIG. 36(A) is a distribution diagram of an electron/current density when X 2 is 40% under OLT stressing conditions
  • FIG. 36(B) is a distribution diagram of an electron/current density when X 2 is 120% under OLT stressing conditions
  • FIG. 36(C) is a distribution diagram of an electron/current density when X 2 is 200% under OLT stressing conditions;
  • FIG. 37 is a view showing electrons and currents along the interface of a semiconductor substrate between the points P 1 and P 2 of FIG. 36 when X 2 is assumed to be 40%, 120%, and 200% under OLT stressing conditions;
  • FIG. 38(A) is a distribution diagram of an electric field intensity when X 2 is 40% under OLT stressing conditions
  • FIG. 38(B) is a distribution diagram of an electric field intensity when X 2 is 120% under OLT stressing conditions
  • FIG. 38(C) is a distribution diagram of an electric field intensity when X 2 is 200% under OLT stressing conditions
  • FIG. 39 is a view showing electric field intensities along the interface of a semiconductor substrate between the points P 1 and P 2 of FIG. 38 when X 2 is assumed to be 40%, 120%, and 200% under OLT stressing conditions;
  • FIG. 40(A) is a distribution diagram of an impact ionization ratio when X 2 is 40% under OLT stressing conditions
  • FIG. 40(B) is a distribution diagram of an impact ionization ratio when X 2 is 120% under OLT stressing conditions
  • FIG. 40(C) is a distribution diagram of an impact ionization ratio when X 2 is 200% under OLT stressing conditions;
  • FIG. 41 is a view showing impact ionization ratios along the interface of a semiconductor substrate between the points P 1 and P 2 of FIG. 40 when X 2 is assumed to be 40%, 120%, and 200% under OLT stressing conditions;
  • FIG. 42 is a plan view schematically showing a configuration of a semiconductor device in Embodiment 7 of the present invention in which angular portions are formed in a part of a trench when viewed in plan view;
  • FIG. 43 is a schematic cross-sectional view along the line XLIII-XLIII of each of FIGS. 42 and 44 ;
  • FIG. 44 is a plan view schematically showing a configuration of the semiconductor device in Embodiment 7 of the present invention in which the angular portions are formed in the entire trench when viewed in plan view;
  • FIG. 45 is a schematic cross-sectional view showing a configuration of a RESURF MOS transistor in which an inclined portion is provided at the source-side sidewall of a trench of a STI structure;
  • FIG. 46 is a schematic cross-sectional view showing a configuration of a RESURF MOS transistor in which an inclined portion is provided at the drain-side sidewall of a trench of a STI structure;
  • FIG. 47 is a schematic cross-sectional view showing a configuration of a RESURF MOS transistor in which an inclined portion is provided at each of the source-side sidewall and the drain-side sidewall of a trench of a STI structure;
  • FIG. 48 is a schematic cross-sectional view showing a configuration of a NON-RESURF MOS transistor in which an inclined portion is provided at each of the source-side sidewall and the drain-side sidewall of a trench of a STI structure;
  • FIG. 49 is a schematic cross-sectional view showing a configuration of an element in which a current is allowed to flow from one side of a STI structure to the other side thereof through a semiconductor region under the STI structure, and an inclined portion is provided at each of the both sidewalls of a trench of the STI structure;
  • FIG. 50 is an enlarged cross-sectional view of each of the trenches of the STI structures shown in FIGS. 45 to 49 , which is for illustrating the cross-sectional shape of the trench;
  • FIG. 51(A) is a distribution diagram of an electron/current density in a comparative example under OLT stressing conditions
  • FIG. 51(B) is a distribution diagram of an electron/current density in the configuration of an inclined type shown in FIG. 45 under OLT stressing conditions
  • FIG. 52(A) is a distribution diagram of an impact ionization ratio in the comparative example under OLT stressing conditions
  • FIG. 52(B) is a distribution diagram of an impact ionization ratio in the configuration of the inclined type shown in FIG. 45 under OLT stressing conditions;
  • FIG. 53 is a view showing the distribution of the impact ionization ratio along the interface of a semiconductor substrate between the points P 11 and P 12 shown in FIG. 52 in each of the configuration of the comparative example and the configuration (of the inclined type) shown in FIG. 45 , each under OLT stressing conditions;
  • FIG. 54(A) is a distribution diagram of an electron/current density in a comparative example under OLT stressing conditions
  • FIG. 54(B) is a distribution diagram of an electron/current density in the configuration of an inclined type shown in FIG. 46 under OLT stressing conditions
  • FIG. 55(A) is a distribution diagram of an impact ionization ratio in the comparative example under OLT stressing conditions
  • FIG. 55(B) is a distribution diagram of an impact ionization ratio in the configuration of the inclined type shown in FIG. 46 under OLT stressing conditions;
  • FIG. 56 is a view showing the distribution of the impact ionization ratio along the interface of a semiconductor substrate between the points P 13 and P 14 shown in FIG. 55 in each of the configuration of the comparative example and the configuration (of the inclined type) shown in FIG. 46 , each under OLT stressing conditions;
  • FIG. 57 is a schematic cross-sectional view showing a configuration in which the sidewalls of a STI structure have stepped shapes, and each of the sidewalls has a plurality of angular portions.
  • Embodiment 1 First, a configuration of a semiconductor device in Embodiment 1 will be described using FIGS. 1 and 2 .
  • an analog/digital mixed chip CH has, e.g., an analog element formation region AN, a logic element formation region LO, a memory element formation region NVM, and a power element formation region PW.
  • an analog element formation region AN formed is an analog device such as, e.g., a resistor.
  • CMOS complementary MOS
  • the logic element formation region LO formed is, e.g., a complementary MOS (CMOS) transistor or the like.
  • CMOS complementary MOS
  • the memory element formation region NVM formed is, e.g., a stacked-gate nonvolatile memory or the like.
  • the power element formation region PW formed is, e.g., an LDMOS transistor or the like.
  • the LDMOS transistor described above primarily has a semiconductor substrate SUB, an n + source region (first region) SO, an n + drain region (second region) DR, a p-type well region (third region) WL, an n-type drift region (fourth region) DRI, a p ⁇ epitaxial region (fifth region) EP, a gate electrode layer GE, and a STI structure TR, BI.
  • the semiconductor substrate SUB is made of, e.g., silicon, and has a trench TR in the main surface thereof.
  • a trench TR in the main surface thereof.
  • a buried insulating film BI In the trench TR, formed is a buried insulating film BI.
  • the trench TR and the buried insulating film BI form a STI structure.
  • the trench TR forming the STI structure has a bottom portion BT, and one wall portion FS and the other wall portion SS which oppose each other.
  • the n + source region SO is formed in the main surface of a portion of the semiconductor substrate SUB located closer to the one wall portion FS of the trench TR.
  • a p + body contact region IR is formed to be adjacent to the n + source region SO.
  • the p-type well region WL is formed to be located under the n + source region SO and the p + body contact region IR, and located in a part of the main surface of the semiconductor substrate SUB interposed between the n + source region SO and the trench TR.
  • the p-type well region WL has a p-type impurity concentration lower than that of the p + body contact region IR, and forms a p-n junction with the n + source region SO.
  • the n + drain region DR is formed in the main surface of a portion of the semiconductor substrate SUB located closer to the other wall portion SS of the trench TR.
  • the n-type drift region DRI is formed in the semiconductor substrate SUB to be located under the n + drain region DR and the trench TR, and located in a part of the main surface of the semiconductor substrate SUB interposed between the trench TR and the n + source region SO.
  • the n-type drift region DRI has an n-type impurity concentration lower than that of the n + drain region DR.
  • the n-type drift region DRI is formed sidewise of the p-type well region WL in adjacent relation thereto, and forms a p-n junction with the p-type well region WL.
  • the p ⁇ epitaxial region EP is formed in the semiconductor substrate SUB so as to be located under each of the n-type drift region DRI and the p-type well region WL.
  • the p ⁇ epitaxial region EP is formed in contact with each of the n-type drift region DRI and the p-type well region WL.
  • the p ⁇ epitaxial region EP forms a p-n junction with the n-type drift region DRI, and a part of the p-n junction is located in a plane (in a plane substantially parallel with the main surface) along the main surface of the semiconductor substrate SUB.
  • the p ⁇ epitaxial region EP has a p-type impurity concentration lower than that of the p-type well region WL.
  • the gate electrode layer GE is formed over the p-type well region WL and the n-type drift region DRI which are located in the main surface of the semiconductor substrate SUB via a gate insulating film GI.
  • the gate electrode layer GE is formed such that the drain-side end portion thereof lies over the buried insulating film BI, whereby a field plate effect using the gate electrode layer GE can be obtained.
  • a source conductive layer SCL is formed over the main surface of the semiconductor substrate SUB so as to be electrically coupled to each of the n + source region SO and the p + body contact region IR.
  • a drain conductive layer DCL is formed over the main surface of the semiconductor substrate SUB so as to be electrically coupled to the n + drain region DR.
  • the angular portions are formed at, e.g., the one wall portion FS of the trench TR closer to the source.
  • the one wall portion FS of the trench TR is formed with, e.g., a projecting angular portions CP 1 A and a depressed angular portion CP 2 A so that the one wall portion FS has a stepped shape when viewed in cross section.
  • the two angular portions CP 1 A and CP 2 A are located between an upper portion UP 1 of the trench TR located in the main surface of the semiconductor substrate SUB and the bottom portion BT thereof. Note that, in the present embodiment, the angular portions located between the upper portion UP 1 and the bottom portion BT of the trench TR do not include an edge portion ED 1 formed at the intersection of the bottom portion BT and the one wall portion FS of the trench TR.
  • Each of the angular portions described above may have a right-angled shape, an obtuse-angled shape, or an acute-angled shape in the cross section of FIG. 2 .
  • the stepped shape of the one wall portion FS of the trench TR will be described using FIG. 3 .
  • the dimension from the upper portion UP 1 of the one wall portion FS of the trench TR to the bottom portion BT thereof in a depth direction (thickness direction) is S 1 and proportions (%) are X 2 and Y 2
  • the angular portion CP 1 A is located at a position (position closer to the main surface of the semiconductor substrate SUB) shallower by S 1 ⁇ Y 2 than the bottom portion BT of the trench TR.
  • junction portion (edge portion) ED 1 between the one wall portion FS and the bottom portion BT is located at the depth S 1 from the main surface of the semiconductor substrate SUB and closer to the drain (closer to the other wall portion SS) by S 1 ⁇ X 2 as measured from the upper portion UP 1 of the trench TR in a lateral direction (direction along the main surface of the semiconductor substrate SUB).
  • the proportion Y 2 is preferably not less than 40% and not more than 80%.
  • FIGS. 4 to 13 a method of manufacturing the semiconductor device according to the present embodiment will be described using FIGS. 4 to 13 .
  • the LDMOS transistor hereinafter referred to as a “SS-LDMOS transistor”
  • FIG. 2 the LDMOS transistor
  • the semiconductor substrate SUB is prepared which has the p ⁇ epitaxial region EP in each of the SS-LDMOS transistor region (first element region), the CMOS transistor region (second element region), and the nonvolatile memory region (third element region).
  • impurity diffusion layers required for the individual elements are formed by ion implantation or the like.
  • the n-type drift region DRI and the p-type well region WL are formed in the SS-LDMOS transistor region.
  • An n-type region WL 1 is formed in the pMOS transistor region, while a p-type region WL 2 is formed in each of the nMOS transistor region and the nonvolatile memory region NVM.
  • a hard mask (insulating film) HM for forming the STI structure is formed over the entire main surface of the semiconductor substrate SUB.
  • a patterned photoresist PR 1 is formed using a typical photoengraving technique. Using the photoresist PR 1 as a mask, anisotropic etching is performed with respect to the hard mask HM. Thereafter, the photoresist mask PR 1 is removed by, e.g., ashing or the like.
  • the hard mask HM is patterned by the etching described above to expose the surface of a part of the semiconductor substrate SUB. Using the patterned hard mask HM as a mask, anisotropic etching is performed with respect to the exposed main surface of the semiconductor substrate SUB.
  • the trench (first trench) TRA for the STI structure is formed in the main surface of the semiconductor substrate SUB in the SS-LDMOS transistor region.
  • a trench (third trench) TRA for the STI structure is also formed in the CMOS transistor region, and a trench (fourth trench) TRA for the STI structure is also formed in the nonvolatile memory region, each by the same step of forming the trench (first trench) TRA in the SS-LDMOS transistor region.
  • Each of these trenches TRA is formed to have a pair of sidewalls opposing each other, and have substantially the same depths.
  • a patterned photoresist PR 2 is formed.
  • the photoresist PR 2 is formed in the SS-LDMOS transistor region to cover the source-side sidewall of the trench (first trench) TRA, and open the drain-side sidewall and the middle portion of the trench (first trench) TRA between the source-side and drain-side sidewalls thereof.
  • the photoresist PR 2 is also formed to open the entire trench (third trench) TRA in the CMOS transistor region, and cover the entire trench (fourth trench) TRA in the nonvolatile memory region.
  • a trench (second trench) TRB is formed to extend deep under the middle portion and the drain-side sidewall of the trench (first trench) TRA in the SS-LDMOS transistor region.
  • the trench (third trench) TRA in the CMOS transistor region becomes the trench TRB extending to substantially the same depth as that of the trench (second trench) TRB in the SS-LDMOS transistor region.
  • the trench (first trench) TRA and the trench (second trench) TRB form the trench TR having the angular portions CP 1 A and CP 2 A forming the stepped shape at the one wall portion FS thereof.
  • the respective depths of the angular portions CP 1 A and CP 2 A are substantially the same as the depth of the trench (fourth trench) TRA in the nonvolatile memory region.
  • the depth of the bottom portion BT of the trench TR in the SS-LDMOS transistor region is substantially the same as the depth of the trench (third trench) TRB in the CMOS transistor region.
  • the photoresist PR 2 is removed by, e.g., ashing or the like.
  • the buried insulating film BI is formed so as to be buried in each of the trenches TR, TRA, and TRB.
  • the gate insulating film GI, the gate electrode layer GE, the source region SO, the drain region DR, and the p + body contact region IR are formed.
  • the gate insulating film GI, the gate electrode layer GE, and source/drain regions SD are formed.
  • the gate insulating film GI, a floating gate electrode FG, an intergate insulating film GB 1 , a control gate electrode CG, and the source/drain regions SD are formed.
  • the semiconductor device according to the present embodiment is manufactured. Next, the operation and effect of the semiconductor device according to the present embodiment will be described in comparison with those of a comparative example shown in FIG. 14 .
  • a semiconductor device according to the comparative example is different from the configuration of the present embodiment shown in FIG. 2 in that the one wall portion FS of the trench TR forming the STI structure is not formed with angular portions. Accordingly, each of the one wall portion FS and the other wall portion SS of the trench TR in the comparative example has a cross-sectional shape which is linear from the upper portion UP 1 of the trench TR to the bottom portion thereof.
  • the configuration of the comparative example is substantially the same as the configuration of the present embodiment shown in FIG. 2 . Therefore, a description thereof is omitted by providing the same components with the same reference numerals.
  • the present inventors examined an amount of degradation of a drain current Ids in an Operating Life Test (OLT) in each of the configuration of the present embodiment shown in FIG. 2 and the configuration of the comparative example shown in FIG. 14 .
  • the result of the examination is shown in FIG. 15 .
  • the OLT test is a method which gives a given stress (voltage or temperature) to an element under measurement, and evaluates the amount of degradation of the drain current Ids and an amount of time-dependent change of a threshold voltage Vth.
  • a stress during a worst-case operation is given to an element having an ON breakdown voltage of not less than 60 V by setting a gate voltage Vg to 3.3 V, and setting a drain voltage Vd to 45 V.
  • FIGS. 16(A) and 16(B) show the distributions of electron/current densities in the respective configurations of the comparative example and the present embodiment shown in FIG. 2 , each under stressing conditions.
  • FIGS. 17(A) and 17(B) show the distributions of compact ionization ratios in the respective configurations of the comparative example and the present embodiment shown in FIG. 2 , each under stressing conditions.
  • X 2 and Y 2 mentioned in the description of FIG. 3 were set to 120% and 40%, respectively.
  • FIG. 18 shows the distribution of the impact ionization ratio along the interface of the semiconductor substrate between the points P 1 and P 2 in each of FIGS. 17(A) and 17(B) .
  • the electron/current density and the impact ionization ratio are high in the vicinity of the edge portion ED 1 of the trench TR in the configuration of the comparative example, and also high in the vicinity of each of the edge portion ED 1 and the angular portion CP 1 A of the trench TR in the configuration of the present embodiment shown in FIG. 2 . It has also been found that the electron/current density and the impact ionization ratio in the vicinity of each of the edge portion ED 1 and the angular portion CP 1 A in the configuration shown in FIG. 2 are lower than the electron/current density and the impact ionization ratio in the vicinity of the edge portion ED 1 in the configuration of the comparative example shown in FIG. 14 .
  • the impact ionization is a phenomenon in which an electron accelerated by an electric field impinges on a crystal lattice to generate an electron-hole pair. It can be considered that, in the comparative example, a current is concentrated on the edge portion ED 1 of the trench TR to increase the electron/current density, and consequently increase the impact ionization ratio. On the other hand, it can be considered that, in the present embodiment, the one wall portion FS of the trench TR is formed into the stepped shape to distribute the current concentration to the edge portion ED 1 and to the projecting angular portion CP 1 A, and accordingly reduce the current concentration and the impact ionization ratio.
  • the present inventors also examined what influence was exerted on electric characteristics by hot carriers generated by the impact ionization described above when they were trapped at the interface of the edge portion ED 1 of the trench TR.
  • the examination was conducted by device simulation in which negative charges are generated at the interface of the edge portion ED 1 of the trench TR to artificially reproduce a state where electrons are trapped.
  • FIG. 19 shows respective Vg-Ids characteristics when charges are generated in the edge portion ED 1 of the trench TR and when charges are not generated therein. As can be seen from the result of FIG. 19 , an ON current showed a tendency to decrease as the minus charges placed in the edge portion ED 1 of the trench TR were larger.
  • the solid lines in the semiconductor substrate of FIG. 20 are the contours of currents/potentials and, among the plurality of contours, the one closer to the trench TR shows the negatively higher current/potential.
  • the Ids degradation in the OLT test is caused by the trapping of hot carries generated by the impact ionization at the interface of the edge portion ED 1 of the trench TR, and is correlated with the impact ionization ratio in the edge portion ED 1 of the trench TR.
  • the current concentration can be reduced by forming the one wall portion FS of the trench TR into the stepped shape as compared to the reduction of the current concentration in the configuration of the comparative example. Therefore, it can be considered that the generation of hot carriers due to the impact ionization and the trapping of electrons at the interface of the edge portion ED 1 of the trench TR are inhibited, and the Ids degradation can be reduced.
  • the configuration of the present embodiment is different from the configuration of Embodiment 1 shown in FIG. 2 in that the other wall portion SS of the trench TR closer to the drain has a stepped shape, while the one wall portion FS of the trench TR closer to the source does not have a stepped shape.
  • the other wall portion SS of the trench TR has a projecting angular portion CP 1 B and a depressed angular portion CP 2 B to show a stepped shape when viewed in cross section.
  • the two angular portions CP 1 B and CP 2 B are located between an upper portion UP 2 of the trench TR located in the main surface of the semiconductor substrate SUB and the bottom portion BT thereof. Note that, in the present embodiment, the angular portions located between the upper portion TP 2 and the bottom portion BT of the trench TR do not include an edge portion ED 2 formed at the intersection of the bottom portion BT and the other wall portion SS of the trench TR.
  • Each of the angular portions described above may have a right-angled shape, an obtuse-angled shape, or an acute-angled shape in the cross section of FIG. 21 .
  • the configuration of the present embodiment is otherwise substantially the same as the configuration of Embodiment 1 described above. Therefore, a description thereof is omitted by providing the same components with the same reference numerals.
  • the stepped shape of the other wall portion SS of the trench TR will be described using FIG. 3 .
  • the dimension from the upper portion UP 2 of the trench TR to the bottom portion BT thereof in the depth direction is S 2 and proportions (%) are X 1 and Y 1
  • the angular portion CP 1 B of the other wall portion SS is located at a position (position closer to the main surface of the semiconductor substrate SUB) shallower by S 2 ⁇ Y 1 than the bottom portion BT of the trench TR.
  • junction portion (edge portion) ED 2 between the other wall portion SS and the bottom portion BT is located at the depth S 2 from the main surface of the semiconductor substrate SUB and closer to the source (closer to the one wall portion FS) by S 2 ⁇ X 1 as measured from the upper portion UP 2 of the trench TR in a lateral direction (direction along the main surface of the semiconductor substrate SUB).
  • the proportion Y 1 is preferably 40%.
  • Y 1 is preferably not less than 60% and not more than 80%.
  • FIGS. 22 and 23 Next, a method of manufacturing the semiconductor device according to the present embodiment will be described using FIGS. 22 and 23 .
  • the same steps as those of Embodiment 1 shown in FIGS. 4 to 9 are performed first.
  • the patterned photoresist PR 2 is formed using a typical photoengraving technique.
  • the photoresist PR 2 is formed in the SS-LDMOS transistor region to cover the drain side of the trench (first trench) TRA, and open the source side and the middle portion of the trench (first trench) TRA between the source side and the drain side thereof.
  • the photoresist PR 2 is also formed to open the entire trench (third trench) TRA in the CMOS transistor region, and cover the entire trench (fourth trench) TRA in the nonvolatile memory region.
  • the trench (second trench) TRB is formed to extend deep under the middle portion and the source-side sidewall of the trench (first trench) TRA in the SS-LDMOS transistor region.
  • the trench (third trench) TRA of the CMOS transistor region becomes the trench TRB extending to substantially the same depth as that of the trench (second trench) TRB in the SS-LDMOS transistor region.
  • the trench (first trench) TRA and the trench (second trench) TRB form the trench TR having the angular portions CP 1 B and CP 2 B forming the stepped shape at the other wall portion SS thereof.
  • the respective depths of the angular portions CP 1 B and CP 2 B are substantially the same as the depth of the trench (fourth trench) TRA in the nonvolatile memory region.
  • the depth of the bottom portion BT of the trench TR in the SS-LDMOS transistor region is substantially the same as the depth of the trench (third trench) TRB in the CMOS transistor region.
  • the photoresist PR 2 is removed by, e.g., ashing or the like.
  • FIGS. 24(A) and 24(B) show the respective distributions of electron/current densities in the comparative example and the present embodiment shown in FIG. 21 , each under stressing conditions.
  • FIGS. 25(A) and 25(B) show the respective distributions of compact ionization ratios in the comparative example and the present embodiment shown in FIG. 21 , each under stressing conditions.
  • X 1 and Y 1 mentioned in the description of FIG. 3 were set to 120% and 40%, respectively.
  • FIG. 26 shows the distribution of the impact ionization ratio along the interface of the semiconductor substrate between the points P 3 and P 4 in each of FIGS. 25(A) and 25(B) .
  • the electron/current density and the impact ionization ratio are high in the vicinity of the edge portion ED 2 of the trench TR in the comparative example, and also high in the vicinity of each of the edge portion ED 2 and the angular portion CP 1 B of the trench TR in the present embodiment. It has also been found that the electron/current density and the impact ionization ratio in the vicinity of each of the edge portion ED 2 and the angular portion CP 1 B in the present embodiment are lower than the electron/current density and the impact ionization ratio in the vicinity of the edge portion ED 2 in the comparative example.
  • the result described above can be considered in the same manner as in Embodiment 1. That is, it can be considered that, in the comparative example, a current is concentrated on the edge portion ED 2 of the trench TR to increase the electron/current density, and consequently increase the impact ionization ratio.
  • the other wall portion SS of the trench TR is formed into the stepped shape to distribute the current concentration to the edge portion ED 2 and to the projecting angular portion CP 1 B, and accordingly reduce the current concentration and the impact ionization ratio.
  • the current concentration can be reduced by forming the other wall portion SS of the trench TR into the stepped shape as compared to the reduction of the current concentration in the configuration of the comparative example. Therefore, it can be considered that the generation of hot carriers due to the impact ionization and the trapping of electrons at the interface of the edge portion ED 2 of the trench TR are inhibited, and the Ids degradation can be reduced.
  • the configuration of the present embodiment is different from the configuration of Embodiment 1 shown in FIG. 2 in that not only the one wall portion FS of the trench TR closer to the source, but also the other wall portion SS of the trench RT closer to the drain have respective stepped shapes.
  • the one wall portion FS of the trench TR closer to the source has the projecting angular portion CP 1 A and the depressed angular portion CP 2 A to show a stepped shape when viewed in cross section.
  • the other wall portion SS of the trench TR closer to the drain has the projecting angular portion CP 1 B and the depressed angular portion CP 2 B to also show a stepped shape when viewed in cross section.
  • the two angular portions CP 1 A and CP 2 A of the one wall portion FS are located between the upper portion UP 1 of the trench TR located in the main surface of the semiconductor substrate SUB and the bottom portion BT thereof.
  • the two angular portions CP 1 B and CP 2 B are located between the upper portion UP 2 of the trench TR located in the main surface of the semiconductor substrate SUB and the bottom portion BT thereof.
  • Each of these angular portions may have a right-angled shape, an obtuse-angled shape, or an acute-angled shape in the cross section of FIG. 27 .
  • the configuration of the present embodiment is otherwise substantially the same as the configuration of Embodiment 1 described above. Therefore, a description thereof is omitted by providing the same components with the same reference numerals.
  • each of the one wall portion FS and the other wall portion SS of the trench TR it is the same as the shape described above in Embodiments 1 and 2 using FIG. 3 .
  • the angular portion CP 1 A is located at a position (position closer to the main surface of the semiconductor substrate SUB) shallower by S 1 ⁇ Y 2 than the bottom portion BT of the trench TR.
  • the junction portion (edge portion) ED 1 between the one wall portion FS having the stepped shape and the bottom portion BT is located at the depth S 1 from the main surface of the semiconductor substrate SUB and closer to the drain (closer to the other wall portion SS) by S 1 ⁇ X 2 as measured from the upper portion UP 1 of the trench TR in a lateral direction (direction along the main surface of the semiconductor substrate SUB).
  • the proportion Y 2 is preferably not less than 40% and not more than 80%.
  • the angular portion CP 1 B of the other wall portion SS is located at a position (position closer to the main surface of the semiconductor substrate SUB) shallower by S 2 ⁇ Y 1 than the bottom portion BT of the trench TR.
  • the junction portion (edge portion) ED 2 between the other wall portion SS having the stepped shape and the bottom portion BT is located at the depth S 2 from the main surface of the semiconductor substrate SUB and closer to the source (closer to the one wall portion FS) by S 2 ⁇ X 1 as measured from the upper portion UP 2 of the trench TR in a lateral direction (direction along the main surface of the semiconductor substrate SUB).
  • the proportion Y 1 is preferably 40%.
  • X 1 is preferably not less than 60% and not more than 80%.
  • FIGS. 28 and 29 Next, a method of manufacturing the semiconductor device according to the present embodiment will be described using FIGS. 28 and 29 .
  • the same steps as those of Embodiment 1 shown in FIGS. 4 to 9 are performed first.
  • the patterned photoresist PR 2 is formed using a typical photoengraving technique.
  • the photoresist PR 2 is formed in the SS-LDMOS transistor region to cover the drain side and the source side of the trench (first trench) TRA, and open the middle portion of the trench (first trench) TRA interposed between the drain side and the source side thereof.
  • the photoresist PR 2 is also formed to open the entire trench (third trench) TRA in the CMOS transistor region, and cover the entire trench (fourth trench) TRA in the nonvolatile memory region.
  • the trench (second trench) TRB is formed to extend deep under the middle portion of the trench (first trench) in the SS-LDMOS transistor region.
  • the trench (third trench) TRA of the CMOS transistor region becomes the trench TRB extending to substantially the same depth as that of the trench (second trench) TRB in the SS-LDMOS transistor region.
  • the trench (first trench) TRA and the trench (second trench) TRB form the trench TR.
  • the trench TR has the angular portions CP 1 A and CP 2 A forming the stepped shape at the one wall portion FS thereof, and the angular portions CP 1 B and CP 2 B forming the stepped shape at the other wall portion SS thereof.
  • the respective depths of the angular portions CP 1 A, CP 2 A, CP 1 B, and CP 2 B are substantially the same as the depth of the trench (fourth trench) TRA in the nonvolatile memory region.
  • the depth of the bottom portion BT of the trench TR in the SS-LDMOS transistor region is substantially the same as the depth of the trench (third trench) TRB in the CMOS transistor region. Thereafter, the photoresist PR 2 is removed by, e.g., ashing or the like.
  • FIGS. 30(A) and 30(B) show the respective distributions of electron/current densities in the comparative example and the present embodiment shown in FIG. 27 , each under stressing conditions.
  • FIGS. 31(A) and 31(B) show the respective distributions of compact ionization ratios in the comparative example and the present embodiment shown in FIG. 27 , each under stressing conditions.
  • X 1 and X 2 mentioned in the description of FIG. 3 were each set to 120%, and Y 1 and Y 2 each mentioned in the description of FIG. 3 were each set to 40%.
  • FIG. 32 shows the distribution of the impact ionization ratio along the interface of the semiconductor substrate between the points P 5 and P 6 in each of FIGS. 31(A) and 31(B) .
  • the electron/current density and the impact ionization ratio in the vicinity of each of the edge portions ED 1 and ED 2 and the angular portions CP 1 A and CP 1 B in the present embodiment are lower than the electron/current density and the impact ionization ratio in the vicinity of each of the edge portions ED 1 and ED 2 in the comparative example.
  • each of the one wall portion FS and the other wall portion SS of the trench TR is formed into the stepped shape to distribute the current concentration to the edge portions ED 1 and ED 2 and to the projecting angular portions CP 1 A and CP 1 B, and accordingly reduce the current concentration and the impact ionization ratio.
  • the current concentration can be reduced by forming each of the one wall portion FS and the other wall portion SS of the trench TR into the stepped shape as compared to the reduction of the current concentration in the configuration of the comparative example. Therefore, it can be considered that the generation of hot carriers due to the impact ionization and the trapping of electrons at the interfaces of the edge portions ED 1 and ED 2 of the trench TR are inhibited, and the Ids degradation can be reduced.
  • the configuration of the present embodiment is different from the configuration of Embodiment 3 shown in FIG. 27 in that an n-type buried region BL is formed, and that the n-type drift (offset) region DRI is formed to extend under the p-type well region WL.
  • the n-type buried region BL is formed in the semiconductor substrate SUB to be located over the p ⁇ epitaxial region EP, and form a p-n junction with the p ⁇ epitaxial region EP.
  • a p ⁇ epitaxial region EPA Over the n-type buried region BL, formed is a p ⁇ epitaxial region EPA.
  • the n-type drift region DRI 1 is formed to extend under the p-type well region WL, and formed to come in contact with the n-type buried region BL. In this manner, the NON-RESURF SS-LDMOS transistor has been formed.
  • the configuration of the present embodiment is otherwise substantially the same as the configuration of Embodiment 3 described above. Therefore, a description thereof is omitted by providing the same components with the same reference numerals.
  • a NON-RESURF SS-LDMOS transistor as described in the present embodiment also, by forming the angular portions CP 1 A, CP 2 A, CP 1 B, and CP 2 B in at least either one of the one wall portion FS and the other wall portion SS of the trench TR, it is possible to reduce the current concentration on the wall portions of the trench TR, and inhibit the degradation of electric characteristics due to the trapping of electrons.
  • the angular portions may also be provided at either one of the one wall portion FS and the other wall portion SS.
  • the trench having the angular portions can also be applied to an element in which a current is allowed to flow into a region of a semiconductor substrate located under the trench. The configuration thereof will be described below using FIG. 34 .
  • the p ⁇ epitaxial region EP is formed in the semiconductor substrate SUB.
  • an n-type region DI is formed to form a p-n junction with the p ⁇ epitaxial region EP.
  • the STI structure In the main surface of the semiconductor substrate SUB over the n-type region DI, formed is the STI structure.
  • the STI structure has the trench TR formed in the main surface of the semiconductor substrate SUB, and the buried insulating film BI to be buried in the trench TR.
  • the one wall portion FS of the trench TR is provided with the angular portions CP 1 A and CP 2 A, while the other wall portion SS thereof is provided with the angular portions CP 1 B and CP 2 B.
  • the shapes and positions of these angular portions are substantially the same as in the configuration of Embodiment 3 so that a description thereof is omitted.
  • an n + region IR 2 having an n-type impurity concentration higher than that of the n-type region DI.
  • an n + region IR 1 having an n-type impurity concentration higher than that of the n-type region DI.
  • the n + region IR 2 is, e.g., a region to which a relatively low voltage is applied, while the n + region IR 1 is, e.g., a region to which a relatively high voltage is applied.
  • a current can be allowed to flow through the region of the semiconductor substrate SUB located under the trench TR, and between the n + region IR 2 and the n + region IR 1 .
  • the angular portions in at least either one of the one wall portion FS and the other wall portion SS of the trench TR, it is possible to reduce the electric field concentration on the wall portions of the trench TR, and inhibit the degradation of electric characteristics due to the trapping of electrons.
  • the angular portions may also be provided at either one of the one wall portion FS and the other wall portion SS.
  • the present inventors examined a preferred shape as the stepped shape of the one wall portion FS of the trench TR in Embodiment 1 shown in FIG. 2 .
  • the content and result of the examination will be described using FIGS. 35 and 37 .
  • the present inventors evaluated X 2 dependence in the stepped shape of the one wall portion FS of the trench TR.
  • the evaluation was performed by setting Y 2 to 40%, and setting X 2 to the three levels of 40%, 120%, and 200%.
  • the result of the evaluation is shown in FIG. 35 .
  • the present inventors also examined an electron/current distribution, the distribution of an electric field intensity, and the distribution of an impact ionization ratio. The results of the examination are shown in FIGS. 36 to 41 .
  • FIGS. 36(A) , 36 (B), and 36 (C) show the respective electron/current distributions when X 2 is 40%, when X 2 is 120%, and when X 2 is 200%, each under OLT stressing conditions.
  • FIG. 37 shows the electron/current distribution along the interface of a semiconductor substrate between the points P 1 and P 2 of each of FIGS. 36(A) to 36(C) under OLT stressing conditions.
  • FIGS. 38(A) , 38 (B), and 38 (C) show the respective distributions of the electric field intensities when X 2 is 40%, when X 2 is 120%, and when X 2 is 200% under OLT stressing conditions.
  • FIGS. 40(A) , 40 (B), and 40 (C) show the respective distributions of the impact ionization ratios when X 2 is 40%, when X 2 is 120%, and when X 2 is 200% under OLT stressing conditions.
  • FIG. 41 shows the distribution of the impact ionization ratio along the interface of the semiconductor substrate between the points P 1 and P 2 of each of FIGS. 40(A) to 40(C) under OLT stressing conditions.
  • the present inventors also performed simulation for examining the impact ionization ratios when the respective values of X 2 and Y 2 in the stepped shape of the one wall portion FS of the trench TR were varied.
  • the result of the simulation is shown below in Table 1.
  • the present inventors also performed simulation for examining the impact ionization ratios when the respective values of X 1 and Y 1 in the stepped shape of the other wall portion SS of the trench TR were varied.
  • the result of the simulation was shown below in Table 2.
  • FIGS. 42 to 44 a two-dimensional layout of the angular portions will be described using FIGS. 42 to 44 .
  • the gate electrode layer GE and the p + body contact region IR are formed to be located on the outer periphery of the drain region DR when viewed in plan view ( FIG. 42 ).
  • the source region SO is formed at a position opposing the drain region DR in the short-side direction (direction indicated by the arrow M in FIG. 42 ) of the drain region DR when viewed in plan view, and is not formed in the element terminal portion RS including a position opposing the drain region DR in the long-side direction (direction indicated by the arrow N in FIG. 42 ) of the drain region DR.
  • the angular portions CP 1 A and CP 2 A are each formed in a part of the trench TR so as to be located between the source region SO and the drain region DR when viewed in plan view.
  • the source region SO and the drain region DR have mutually equal dimensions in the long-side direction (direction indicated by the arrow N in FIG. 42 ) when viewed in plan view.
  • the angular portions CP 1 A and CP 2 A are formed at respective positions opposing the source region SO and the drain region DR in the short-side direction between the source region SO and the drain region DR.
  • the angular portions CP 1 A and CP 2 A are not formed in the element terminal portion RS, and have the same dimensions as those of the source region SO and the drain region DR in the long-side direction (the direction indicated by the arrow N in FIG. 42 ).
  • the source region SO is formed to be located on the outer periphery of the drain region DR when viewed in plan view ( FIG. 44 ). That is, the angular portions CP 1 A and CP 2 A are formed in the entire trench TR so as to be located between the source region SO and the drain region DR when viewed in plan view.
  • the source region SO is formed in each of the long-side direction (direction indicated by the arrow N in FIG. 42 ) and the short-side direction (direction indicated by the arrow M in FIG. 42 ) of the drain region DR when viewed in plan view.
  • the angular portions CP 1 A and CP 2 A are each formed to surround the periphery of the drain region DR between the source region SO and the drain region DR.
  • the angular portions CP 1 A and CP 2 A are each formed also in the element terminal portion RS.
  • each of the angular portions CP 1 A and CP 2 A of the trench TR is preferably disposed in a region interposed between the source region SO and the drain region DR when viewed in plan view.
  • Each of the angular portions CP 1 B and CP 2 B provided at the other wall portion SS of the trench TR is also preferably provided in the region interposed between the source SO and the drain DR when viewed in plan view, similarly to the angular portions CP 1 A and CP 2 A described above.
  • the angular portions may also be provided so as to form an inclined portion at each of the one wall portion FS and the other wall portion SS of the trench TR when viewed in cross section.
  • a configuration in which each of the one wall portion FS and the other wall portion SS of the trench TR has angular portions which form the inclined portion will be described below using FIGS. 45 to 49 .
  • the configuration is obtained by replacing the stepped shape of the one wall portion FS of the trench TR in Embodiment 1 shown in FIG. 2 with a shape having an inclined portion.
  • the one wall portion FS of the trench TR has a projecting angular portion CPA.
  • the wall portion between the angular portion CPA and the bottom portion BT forms an inclined portion INC 1 inclined with respect to the wall portion between the angular portion CPA and the upper portion UP 1 .
  • the angle ⁇ 1 of the projecting angular portion CPA is an obtuse angle.
  • the inclined portion INC 1 is linear when viewed in cross section.
  • the configuration is obtained by replacing the stepped shape of the other wall portion SS of the trench TR in Embodiment 2 shown in FIG. 21 with a shape having an inclined portion.
  • the other wall portion SS of the trench TR has a projecting angular portion CPB.
  • the wall portion between the angular portion CPB and the bottom portion BT forms an inclined portion INC 2 inclined with respect to the wall portion between the angular portion CPB and the upper portion UP 2 .
  • the angle ⁇ 2 of the projecting portion CPB is an obtuse angle.
  • the inclined portion INC 2 is linear when viewed in cross section.
  • the configuration is obtained by replacing the stepped shape of each of the one wall portion FS and the other wall portion SS of the trench TR in Embodiment 3 shown in FIG. 27 with a shape having an inclined portion.
  • the one wall portion FS of the trench TR has the projecting angular portion CPA.
  • the wall portion between the angular portion CPA and the bottom portion BT forms the inclined portion INC 1 inclined with respect to the wall portion between the angular portion CPA and the upper portion UP 1 .
  • the other wall portion SS of the trench TR has the projecting angular portion CPB.
  • the wall portion between the angular portion CPB and the bottom portion BT forms the inclined portion INC 2 inclined with respect to the wall portion between the angular portion CPB and the upper portion UP 2 .
  • Each of the angle ⁇ 1 of the projecting angular portion CPA and the angle ⁇ 2 of the projecting angular portion CPB is an obtuse angle.
  • Each of the inclined portions INC 1 and INC 2 is linear when viewed in cross section.
  • the configuration is obtained by replacing the stepped shape of each of the one wall portion FS and the other wall portion SS of the trench TR in Embodiment 4 shown in FIG. 33 with the shape having the inclined portion.
  • the angular portion CPA of the one wall portion FS of the trench TR and the angular portion CPB of the other wall portion SS thereof have the same shapes as those of the angular portion CPA and the angular portion CPB in the configuration of FIG. 47 , respectively, so that a description thereof is omitted.
  • the configuration is obtained by replacing the stepped shape of each of the one wall portion FS and the other wall portion SS of the trench TR in Embodiment 5 shown in FIG. 34 with the shape having the inclined portion.
  • the angular portion CPA of the one wall portion FS of the trench TR and the angular portion CPB of the other wall portion SS thereof have the same shapes as those of the angular portion CPA and the angular portion CPB in the configuration of FIG. 47 , respectively, so that a description thereof is omitted.
  • the present inventors reproduced a stressed condition under an OLT test by device simulation, and made a comparison between the internal state of the element in the comparative example shown in FIG. 14 and that of the element in the present embodiment.
  • the content and result of the comparative examination will be described using FIGS. 50 to 56 .
  • the angular portion CPA is located at a position (position closer to the main surface of the semiconductor substrate SUB) shallower by S 1 ⁇ Y 2 than the bottom portion BT of the trench TR.
  • junction portion (edge portion) ED 1 between the one wall portion FS and the bottom portion BT is located at the depth S 1 from the main surface of the semiconductor substrate SUB and closer to the drain (closer to the other wall portion SS) by S 1 ⁇ X 2 as measured from the upper portion UP 1 of the trench TR in a lateral direction (direction along the main surface of the semiconductor substrate SUB).
  • the angular portion CPB is located at a position (position closer to the main surface of the semiconductor substrate SUB) shallower by S 2 ⁇ Y 1 than the bottom portion BT of the trench TR.
  • junction portion (edge portion) ED 2 between the other wall portion SS and the bottom portion BT is located at the depth S 2 from the main surface of the semiconductor substrate SUB and closer to the source (closer to the one wall portion FS) by S 2 ⁇ X 1 as measured from the upper portion UP 2 of the trench TR in a lateral direction (direction along the main surface of the semiconductor substrate SUB).
  • FIGS. 51(A) and 51(B) show the respective distributions of electron/current densities in the configuration of the comparative example and the configuration of FIG. 45 , each under stressing conditions.
  • FIGS. 52(A) and 52(B) show the respective distributions of impact ionization ratios in the configuration of the comparative example and the configuration of FIG. 45 , each under stressing conditions.
  • FIG. 53 shows the distribution of the impact ionization ratio along the interface of the semiconductor substrate between the points P 11 and P 12 in each of FIGS. 52(A) and 52(B) .
  • the electron/current density and the impact ionization ratio are high in the vicinity of the edge portion ED 1 of the trench TR in the configuration of the comparative example, and also high in the vicinity of each of the edge portion ED 1 and the angular portion CPA of the trench TR in the configuration of FIG. 45 . It has also been found that the electron/current density and the impact ionization ratio in the vicinity of each of the edge portion ED 1 and the angular portion CPA in the configuration of FIG. 45 are lower than the electron/current density and the impact ionization ratio in the vicinity of the edge portion ED 1 in the configuration of the comparative example.
  • the current concentration can be reduced by forming the one wall portion FS of the trench TR into the shape having the inclined portion as compared to the reduction of the current concentration in the configuration of the comparative example. Therefore, it can be considered that the generation of hot carriers due to the impact ionization and the trapping of electrons at the interface of the edge portion ED 1 of the trench TR are inhibited, and the Ids degradation can be reduced.
  • FIGS. 54(A) and 54(B) show the respective distributions of electron/current densities in the configuration of the comparative example and the configuration of FIG. 46 , each under stressing conditions.
  • FIGS. 55(A) and 55(B) show the respective distributions of impact ionization ratios in the configuration of the comparative example and the configuration of FIG. 46 , each under stressing conditions.
  • FIG. 56 shows the distribution of the impact ionization ratio along the interface of the semiconductor substrate between the points P 11 and P 12 in each of FIGS. 55(A) and 55(B) .
  • the electron/current density and the impact ionization ratio are high in the vicinity of the edge portion ED 2 of the trench TR in the configuration of the comparative example, and also high in the vicinity of each of the edge portion ED 2 and the angular portion CPB of the trench TR in the configuration of FIG. 46 . It has also been found that the electron/current density and the impact ionization ratio in the vicinity of each of the edge portion ED 2 and the angular portion CPB in the configuration of FIG. 46 are lower than the electron/current density and the impact ionization ratio in the vicinity of the edge portion ED 2 in the configuration of the comparative example.
  • the current concentration can be reduced by forming the other wall portion SS of the trench TR into the shape having the inclined portion as compared to the reduction of the current concentration in the configuration of the comparative example. Therefore, it can be considered that the generation of hot carriers due to the impact ionization and the trapping of electrons at the interface of the edge portion ED 2 of the trench TR are inhibited, and the Ids degradation can be reduced.
  • the present inventors examined respective preferred shapes as the shape of the inclined portion of the one wall portion FS of the trench TR in the configuration shown in FIG. 45 and the shape of the inclined portion of the other wall portion SS of the trench TR in the configuration shown in FIG. 46 .
  • the content and result of the examination will be described below.
  • the present inventors performed simulation for examining the impact ionization ratios when the respective values of X 2 and Y 2 in the one wall portion FS of the trench TR of the configuration shown in FIG. 45 were varied.
  • the result of the simulation is shown below in Table 3.
  • the present inventors Based on the definition of the shape of the trench in FIG. 50 , the present inventors also performed simulation for examining the impact ionization ratios when the respective values of X 1 and Y 1 in the other wall portion SS of the trench TR of the configuration shown in FIG. 46 were varied. The result of the simulation is shown below in Table 4.
  • each of the one wall portion FS and the other wall portion SS of the trench TR may also have a plurality of two or more stepped portions.
  • each of the one wall portion FS and the other wall portion SS of the trench TR has the plurality of stepped portions, it follows that a plurality of pairs of projecting angular portions CP 1 and depressed angular portions CP 2 are formed at each of the one wall portion FS and the other wall portion SS.
  • Each of the one wall portion FS and the other wall portion SS of the trench TR in each of Embodiments 8 and 9 may have a plurality of the angular portions for forming the inclined portion.
  • each of the one wall portion FS and the other wall portion SS of the trench TR when viewed in cross section may also be a shape obtained by appropriately combining the stepped shape in each of Embodiments 1 to 7 with the inclined portion in each of Embodiments 8 and 9.
  • Such a configuration according to the present invention having angular portions in at least either one of the one wall portion FS and the other wall portion SS may also be applied to an Insulated Gate Bipolar Transistor (IGBT) in which a p-type emitter region is provided in place of the n + drain region DR of the LDMOS transistor.
  • IGBT Insulated Gate Bipolar Transistor
  • the configuration according to the present invention may also be applied not only to a MOS transistor having a gate insulating film formed of a silicon dioxide film, but also to a Metal Insulator Semiconductor (MIS) transistor.
  • MIS Metal Insulator Semiconductor
  • Each of the elements shown above may also have a configuration in which the p-type and n-type conductivities are reversed.

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US9356138B2 (en) 2014-02-27 2016-05-31 Renesas Electronics Corporation Semiconductor device
US9755069B2 (en) 2014-02-27 2017-09-05 Renesas Electronics Corporation Semiconductor device
US20190189743A1 (en) * 2017-12-15 2019-06-20 Infineon Technologies Ag Planar Field Effect Transistor
US10573743B2 (en) 2018-03-16 2020-02-25 Kabushiki Kaisha Toshiba Semiconductor device
US11132469B2 (en) * 2019-04-17 2021-09-28 Micron Technology, Inc. Suspicious activity monitoring memory system

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JP5769915B2 (ja) 2015-08-26

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